Lecture 6(2)

Lecture 6(2) - How Does Photosynthesis Convert Light...

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Unformatted text preview: How Does Photosynthesis Convert Light Energy into Chemical Energy? Two systems of electron transport: • Noncyclic electron transport—produces NADPH and ATP • Cyclic electron transport—produces ATP only How Does Photosynthesis Convert Light Energy into Chemical Energy? Noncyclic electron transport: Light energy is used to oxidize water (Oxygenic photosynthesis)→ O2, H+, and electrons. AKer excitaLon by light, Chl+ is an unstable molecule and seeks electrons. Chl+ is a strong oxidizing agent and takes electrons from water, spliOng the water molecule. How Does Photosynthesis Convert Light Energy into Chemical Energy? Two photosystems required in noncyclic electron transport. The photosystems complement each other and enhance photosynthesis. How Does Photosynthesis Convert Light Energy into Chemical Energy? Photosystem I • Light energy reduces NADP+ to NADPH • ReacLon center has chlorophyll a molecules: P700—absorb in the 700nm range Figure 10.10 Noncyclic Electron Transport Uses Two Photosystems (Part 2) How Does Photosynthesis Convert Light Energy into Chemical Energy? Photosystem II • Light energy oxidizes water → O2, H+, and electrons. • ReacLon center has chlorophyll a molecules P680—absorb at 680nm. Figure 10.10 Noncyclic Electron Transport Uses Two Photosystems (Part 1) How Does Photosynthesis Convert Light Energy into Chemical Energy? Cyclic electron transport only makes ATP—an electron from an excited chlorophyll molecule cycles back to the same chlorophyll molecule. Cyclic electron transport begins and ends in photosystem I. Released energy is stored and can be used to form ATP. Figure 10.11 Cyclic Electron Transport Traps Light Energy as ATP How Does Photosynthesis Convert Light Energy into Chemical Energy? Photophosphoryla4on: Light ­driven producLon of ATP—a chemiosmoLc mechanism H+ is transported via electron carriers across the membrane—creaLng an electrochemical gradient. Oxygenic Photosynthesis Figure 17.19 Figure 10.12 Chloroplasts Form ATP ChemiosmoLcally (Part 1) Figure 10.12 Chloroplasts Form ATP ChemiosmoLcally (Part 2) Electron flow • Oxygenic photosynthesis – Electron flow involves two disLnct, but interconnected, photochemical reacLons – Use light to generate ATP and NADPH • Anoxygenic photosynthesis – Electron flow involves one primary photochemical reacLon Anoxygenic photosynthesis NADPH H2A e- + H+ Electron flow in anoxygenic phototrophs • Cyclic photophosphorylaLon – Involves a single Photosystem – Synthesis of ATP occurs as a result of the proton moLve force • Reverse electron flow – Energy requiring process – Electrons are forced backward against the thermodynamic gradient to reduce NAD+ to NADH Anoxygenic photosynthesis NADPH H2A e- + H+ Carbon Flow Chapter 9 and 10 Carbon Sources Heterotrophs: Cells that get carbon from organic compounds Autotrophs: Cells that get carbon from inorganic compounds (CO2) Glucose a common carbon source Fuels: Molecules whose stored energy can be released for use. A common fuel in organisms is glucose. Other molecules are first converted into glucose or other intermediate compounds. How Does Glucose OxidaLon Release Chemical Energy? Sugars, are oxidized to pyruvate by a series of metabolic redox and rearrangement reacLons; There are several ways Hexoses can be oxidized to Pyruvate, a common pathway is called glycolysis. 1 glucose (hexose) 2 pyruvate This is an anaerobic provess How Does Glucose OxidaLon Release Chemical Energy? If O2 is present (aerobic) glycolysis is followed by three pathways of cellular respira4on: • Pyruvate oxidaLon • Citric acid cycle • Electron transport chain If O2 is not present, pyruvate from glycolysis is metabolized by fermenta4on. Figure 9.4 Energy ­Producing Metabolic Pathways What Are the Aerobic Pathways of Glucose Metabolism? Glycolysis takes place in the cytosol: • Converts glucose into pyruvate • Produces a small amount of energy • Generates no CO2 OxidaLon of Glucose Glycolysis involves ten enzyme ­catalyzed reacLons. Energy ­invesLng reacLons require ATP. Energy ­harvesLng reacLons yield NADH and ATP. Net Results in: 2 molecules of pyruvate 2 molecules of ATP (SLP) 2 molecules of NADH OxidaLon of Glucose What else do you get? Precursers: carbon skeltons to build cell material. Glucose-6-Phosphate Fructose-6-Phosphate Pentose-5-Phosphates (pentose cycle) Triose-Phosphate Glycerol-Phosphate Glyceraldehyde Phosphate Phosphoenol pyruvate All necessary for the building of monomers Substrate level phosphorylaLon Phosphoryla4on: addiLon of a phosphate group. Enzyme ­catalyzed transfer of a phosphate group from a donor to ADP to form ATP is called substrate ­level phosphoryla4on. Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 1) Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 2) Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 3) Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 4) Redox ReacLon Substrate level phosphorylaLon Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 5) Figure 9.5 Glycolysis Converts Glucose into Pyruvate (Part 6) Substrate level phosphorylaLon Summary: OxidaLon of Glucose Glycolysis involves ten enzyme ­catalyzed reacLons. Energy ­invesLng reacLons require ATP. Energy ­harvesLng reacLons yield NADH and ATP. Net Results in: 2 molecules of pyruvate 2 molecules of ATP 2 molecules of NADH What do cells do with NADH? If they have an electron transport chain (e ­TC) then NADH can be reoxidezed to NAD+ via e ­TC, and ATP is synthesized. But if no e ­TC, then what? Cells must reoxidize NADH to NAD+ or die. In the absence of an e-TC, cells can reduce oxidized organic compounds by a process called Fermentation. How Is Energy Harvested from Glucose in the Absence of Oxygen? Without an e ­TC, ATP can be produced by glycolysis and NADH reoxidized by fermentaLon. FermentaLon occurs in the cytosol, to regenerate NAD+. Pyruvate from glycolysis is reduced by NADH. How Is Energy Harvested from Glucose in the Absence of Oxygen? Lac4c acid fermenta4on: • Occurs in microorganisms, some muscle cells • Pyruvate is the electron acceptor • Lactate is the product and can build up Figure 9.11 LacLc Acid FermentaLon LacLc acid fermentaLon occurs in vertebrate muscles when O2 is limited. This is why your muscles are sore aKer exercising ! How Is Energy Harvested from Glucose in the Absence of Oxygen? Alcoholic fermenta4on: • Bacteria, Yeasts and some plant cells • Requires two enzymes to metabolize pyruvate to ethanol • Acetaldehyde is reduced by NADH + H+, producing NAD+ and glycolysis conLnues Figure 9.12 Alcoholic FermentaLon Something a bit more closer to home! How Is Energy Harvested from Glucose in the Absence of Oxygen? Bacteria are capable of fermenLng a wide range of organic compounds and produce a vast array of fermentaLon products. Name some? Pyruvate to Acetate Pyruvate Oxida4on: • Links glycolysis and the citric acid cycle; • Pyruvate is oxidized to acetate and CO2 is released • NAD+ is reduced to NADH, capturing energy. • Some energy is stored by combining acetate and Coenzyme A (CoA) to form acetyl CoA Figure 9.7 Pyruvate OxidaLon and the Citric Acid Cycle (Part 1) How Does Glucose OxidaLon Release Chemical Energy? Acetyl CoA is the starLng point of the eight –reacLon citric acid cycle: • Inputs: acetyl CoA, water and electron carriers NAD+, FAD, and GDP • Energy released is captured by ADP and electron carriers NAD+, FAD, and GDP • Outputs: CO2, reduced electron carriers, and GTP, which converts ADP to ATP How Does Glucose OxidaLon Release Chemical Energy? The citric acid cycle is in steady state: The concentraLons of the intermediates don’t change. The cycle conLnues when starLng materials are available: • Acetyl CoA • Reoxidized electron carriers Clicker QuesLon Which of the following are Oxidized electron carriers: A.NAD B. FADH C. GTP D.NADH+ E. All of the above Figure 9.7 Pyruvate OxidaLon and the Citric Acid Cycle (Part 2) Acetate Changes in Free Energy During Glycolysis and the Citric Acid Cycle How Does Glucose OxidaLon Release Chemical Energy? The electron carriers that are reduced during the citric acid cycle must be reoxidized to take part in the cycle again. OxidaLve phosphorylaLon—If an e ­TC is present FermentaLon—if no e ­TC is present (or no terminal e ­ acceptor) How Is Energy Harvested from Glucose in the Absence of Oxygen? Cellular respiraLon yields more energy than fermentaLon per glucose molecule. • Glycolysis plus fermentaLon = 2 ATP • Glycolysis plus cellular respiraLon up to 32 ATPs Figure 9.13 Cellular RespiraLon Yields More Energy Than FermentaLon HOME WORK Home work is to translate these lyrics into scientific reality. Home work example Glucose -- ah, sugar sugar -You are my favorite fuel From the blood-borne substrate pool. Glucose -- monosaccharide sugar -You're sweeter than a woman's kiss 'Cause I need you for glycolysis. I just can't believe the way my muscles take you in. (For you, they'll open the door.) All it takes is a little bit of insulin (To upregulate GLUT4). Summary Heterotrophs get cellular carbon from other organic molecules for biosynthesis. They use Glycolysis to oxidize Glucose to pyruvate, producing a variety of 3 ­Carbon units. They can then oxidize pyruvate acetate, a 2 ­carbon unit. They can further oxidize acetate to CO2 via the TCA cycle. A major point to glycolysis and TCA is to supply the cells with precursors so cells can increase biomass. These intermediates are the building blocks to construct, amino acids, nucleoLdes, and other cellular components. Summary 2 Most reacLons in Glycolysis are freely reversible. For those that are not, alternaLve reacLons can occur. Glycolysis can be run in reverse to form Glucose, gluconeogenesis. Glucose can be stored in polymers such as glycogen, starch, and cellulose. What about Autotrophs? Autotophs can get cellular carbon from CO2, how does that work? Some cells can “fix” or reduce CO2 and produce glucose. This costs both ATP and reducing power, NADPH. Plants do this, many bacteria do this. All know phototrophs can do it. How Is Chemical Energy Used to Synthesize Carbohydrates? CO2 fixaLon: CO2 is reduced to carbohydrates. Enzymes use the energy in ATP and NADPH to reduce CO2. ProducLon of ATP and NADPH is light ­ dependent; therefore CO2 fixaLon must also take place in the light. How Is Chemical Energy Used to Synthesize Carbohydrates? The pathway of CO2 fixaLon is called the Calvin cycle. CO2 is first added to an acceptor molecule—5 ­C RuBP; the 6 ­C compound immediately breaks down into two molecules of 3PG. The enzyme catalyzing the intermediate formaLon is rubisco—ribulose bisphoshate carboxylase/oxygenase—the most abundant protein in the world. How Is Chemical Energy Used to Synthesize Carbohydrates? The Calvin cycle consists of 3 processes: • Fixa2on of CO2 • Reduc2on of 3PG to G3P • Regenera2on of RuBP Figure 10.15 The Calvin Cycle (Part 2) How Is Chemical Energy Used to Synthesize Carbohydrates? G3P: Glyceraldehyde 3 ­phosphate is the product of the Calvin cycle. Most is recycled into RuBP; the rest is used to make sugars or stored starch. How Does Photosynthesis Interact with Other Pathways? Photosynthesis and respiraLon are closely linked through the Calvin cycle. G3P is important: • Some takes part in glycolysis and cellular respiraLon for energy, or can make other compounds • Some is involved in gluconeogenesis, the reverse of glycolysis, supplying nonphotosyntheLc parts Figure 10.20 Metabolic InteracLons in a Plant Cell (Part 1) Figure 10.20 Metabolic InteracLons in a Plant Cell (Part 2) Figure 10.21 Energy Losses During Photosynthesis How Are Metabolic Pathways Interrelated and Regulated? Catabolism (breakdown) and anabolism (biosynthesis) are integrated and Lghtly coupled: NegaLve and posiLve feedback controls ConcentraLons of biochemical molecules remain constant (e.g., glucose concentraLon in blood) Figure 8.14 Metabolic Pathways How Are Metabolic Pathways Interrelated and Regulated? Glycolysis, the citric acid cycle (TCA), and the respiratory chain are subject to allosteric regula,on of key enzymes. Figure 9.15 RegulaLon by NegaLve and PosiLve Feedback Figure 9.16 Allosteric RegulaLon of Glycolysis and the Citric Acid Cycle (Part 1) How Are Metabolic Pathways Interrelated and Regulated? The main control point in glycolysis is phosphofructokinase—allosterically inhibited by ATP. The main control point in the citric acid cycle is isocitrate dehydrogenase—inhibited by NADH + H+ and ATP. How Are Metabolic Pathways Interrelated and Regulated? Another control point, if ATP levels are high: AccumulaLon of citrate diverts acetyl CoA to fapy acid synthesis, for storage. Fapy acids may be metabolized later to produce more acetyl CoA. Lecture 6 Carbon and other Key required chemicals: N, P, S Building a cell • Think of cell’s goal as division. + Input into system Energy to make macromolecules Chemical Chemical constitutents constitutents + Environmental scrutiny Output: Two cells Energy source changes in form + Depletion of Chemical Depletion Constitutents in media Constitutents Building a cell Cells need 1. Energy and reducing power Chemotroph vs Phototroph Organotroph vs Lithotroph 1. Carbon precursors Autotroph vs Organotroph 2. Other basic compounds: Nitrogen, Hydrogen, Oxygen (not talking about respiraLon), Sulfur, Phosphorus, etc. Autotrophy Heterotrophy Building a cell Need carbon precursors to build a cell: 2 carbon units 3 carbon units 4 carbon units Formed during “central metabolism” 5 carbon units 6 carbon units Hydrogen, Oxygen & Nitrogen Hydrogen: – Obtained from H2O, organic compounds Oxygen: – Obtained from H2O, O2, organic compounds Nitrogen: – Nitrogen=12% of cellular dry weight – Ways microbes acquire nitrogen: • Organic sources: proteins, etc • Inorganic: Most bacteria can uLlize ammonia as the sole nitrogen source • Nitrogen gas (N2): Nitrogen ­fixing bacteria Nitrogen Most cells incorporate reduced nitrogen: NH4+ or organic Nitrogen in the form of R ­NH3 However, microbes can use all forms of Nitrogen, reduced, oxidized and in between; thus giving us the Nitrogen Cycle Nitrogen is required for proteins, nucleic acids, vitamins, cofactors, etc…. Nitrogen Cycle RespiraLon Nitrogen fixaLon (Nitrogen reducLon) Nitrogen oxidaLon Other Macronutrients: Phosphorus & Sulfur • Phosphorus: – Obtained from organic & inorganic phosphate – Component of Nucleic Acids and phospholipids • Sulfur – Obtained from inorganic sulfate or sulfide – Chemolithotrophs • Can obtain from elemental sulfur – Component of two amino acids (met and cys) and some vitamins PuOng all together Summary: Chemorganotrophic metabolism Summary: Chemolithotrophic metabolism Summary: Phototrophic metabolism ...
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This note was uploaded on 10/11/2011 for the course BIS 2A taught by Professor Grossberg during the Summer '08 term at UC Davis.

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